The PostgreSQL 7.3.21 sources can be obtained by anonymous FTP from ftp://ftp.postgresql.org/pub/postgresql-7.3.21.tar.gz. Use a mirror if possible. After you have obtained the file, unpack it:
gunzip postgresql-7.3.21.tar.gz tar xf postgresql-7.3.21.tar
This will create a directory postgresql-7.3.21 under the current directory with the PostgreSQL sources. Change into that directory for the rest of the installation procedure.
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